Aims: To examine the role of attention deficit/hyperactivity disorder (ADHD) and impulsive personality disorders in nicotine addiction severity among treatment-seeking substance use disorder (SUD) patients. Methods: In a cross-sectional study, we examined data from the second International ADHD in Substance Use Disorders Prevalence Study (IASP-2) on 402 adults in SUD treatment from Puerto Rico, Hungary, and Australia using diagnostic interviews for ADHD, antisocial (ASP) and borderline (BPD) personality disorders, and the self-report Fagerström Test of Nicotine Dependence (FTND). We compared SUD patients with and without ADHD on nicotine addiction severity. We tested direct and indirect pathways from ADHD to nicotine addiction and mediation through ASP and BPD. Results: Overall, 81.4% of SUD patients reported current cigarette smoking. SUD patients with ADHD had higher FTND scores and smoked more cigarettes than those without ADHD, with an earlier onset and more years of smoking. ASP mediated the effect of ADHD on all aspects of nicotine addiction severity, whereas BPD did so only on some aspects of nicotine addiction severity. Conclusions: SUD patients with comorbid ADHD show more severe nicotine addiction than those without, which is largely explained by comorbid impulsive personality disorders. In SUD patients, it is important to screen for adult ADHD and other psychiatric disorders, especially those with impulse control deficits such as ASP and BPD.